This invention relates to a semiconductor laser made of AlGaInP or similar material whose lateral mode is controlled.
A semiconductor laser emitting light at a wavelength of visible light of 700 nm or less is noticed as a light source for use in optical disk, laser beam printer, bar code reader and others. Above all, the double heterostructure semiconductor laser using GaAs as substrate, Ga.sub.0.multidot.5 In.sub.0.multidot.5 P (hereinafter abbreviated GaInP) or (Al.sub.a Ga.sub.1-x).sub.0.multidot.5 In.sub.0.multidot.5 P (hereinafter abbreviated AlGaInP) for lattice matching therewith as active layer, and AlGaInP as cladding layer is prospective as the material for visible semiconductor laser because it can emit light of the shortest wavelength among the group III-V compound semiconductors having lattice matching with GaAs.
FIGS. 7(a)-7(e) show a sectional structure of manufacturing steps of a conventional lateral mode controlled AlGaInP semiconductor laser. First, as shown in FIG. 7 (a), on the surface of an n-type GaAs substrate 701 having the (100) plane as the principal plane, n-type AlGaInP cladding layer 702, GaInP active layer 703, p-type AlGaInP cladding layer 704, and p type GaInP buffer layer 705 are sequentially formed by crystal growth by the MOVPE (metal organic vapor phase epitaxy) method. Next, using an SiO.sub.2 film 706 formed in stripes in the &lt;011&gt; direction as the mask, a p-type GaInP buffer layer 705 is etched by reactive ion etching (RIE) using, for example, CCl.sub.4 gas, and the p-type AlGaInP cladding layer 704 is etched in sulfuric acid at 40.degree. C., for example, so that the result is as shown in FIG. 7 (b). Furthermore, using the SiO.sub.2 film 706 as the mask, an n-type GaAs current blocking layer 707 is selectively formed by crystal growth by the MOVPE method, and it becomes as shown in FIG. 7 (c). After removing the SiO.sub.2 film 706 used as the mask for selective growth, a p-type GaAs contact layer 708 is formed on the entire surface by crystal growth by the MOVPE method, and stripes are buried as shown in FIG. 7 (d). Finally, on the surface, a p-type ohmic contact layer 709 made of Au/Zn/Au is formed, and the back surface is polished and etched to make the substrate thin, and an n-type ohmic contact layer 710 made of Au--Ge/Ni/Au is formed, and thereby a conventional lateral mode controlled AlGaInP semiconductor laser is completed as shown in FIG. 7 (e).
In this conventional laser, the n-type GaAs current blocking layer 707 electrically serves the role of a current confinement layer, and optically plays the role of absorption type anti-guided layer because it has the refractive index larger than that of the p-type AlGaInP cladding layer 704 and absorbs the light emitted by the GaInP active layer 703. Accordingly, this conventional lateral mode controlled AlGaInP semiconductor laser oscillates at a lower threshold value.
In such conventional lateral mode controlled AlGaInP semiconductor laser, although the lateral mode is controlled, the light due to refractive index in the direction parallel to the active layer is not confined, and the gain guide remains strongly, and therefore the wave front of the guided wave in the direction parallel to the active layer is bent, which results in a large astigmatism. Therefore, when attempted to apply the conventional lateral mode controlled AlGaInP semiconductor laser in optical appliances, the scope of application is limited because it is impossible to spread the laser light into parallel light or focus into one spot by an ordinary convex lens alone.
Besides, since the n-type GaAs current blocking layer absorbs the light emitted in the active layer, it is a loss to the light for guiding the active layer, and it leads to the problem of increase of the oscillation threshold value for the portion of this loss.